I just saw yesterday were some guys leave an long tail kn the knot of the first fly and tie the dropper to that. I have always tied the second fly to the shank of the second. Is there a preferred way, or are they both the same? I think the first way would be easier/faster during initial setup, but when changing flies on the river, would be about the same .

Neil,
I think it is just a matter of preference and fly selection. There are numerous setups that I have seen printed. The main issue in my view is focusing on the intention of throwing multiple setups. Most people try to address multiple levels of a hatch. Others do it to give them more opportunities to catch fish by addressing two different flies. However; I have found that this setup does not work well for me on tailwaters. It usually detracts from my presentation and it is more visible to trout. Plus, I always end up getting wind knots and tangles from my wild casts.

Thanks Shawn, I agree it does tend to get me tangled more. I guess maybe I should focus on presenting one fly better then later try two. I did fish two on tuesday, but the only fish I caught were on the upper fly.

Edit: Posted prior to Shawns response:

Also, how do you gauge distance between flies? Water depth? Always the same? What about dry/emerger riggs. Should the dropper be closer than with double nymph?

The only time I use a double nymph rig is in deeper water and I am essentially using the first fly (usually a beadhead tied closest to the leader) to sink the second fly. Basically it is acting as a split shot with a hook that occasionally attracts catches fish. I have read that, like adding shot, the double rig can get you a better drift.

I usually tie the dropper to the hook bend of the first fly with a pitzen knot becasue I can tie that one in the air and then slip it over the hook and tighten. While I do think that having line tied to the hook makes hookups more difficult, I just fond this method a lot easier than others such as tying to the eye of the first nymph.

When I actually see trout feeding I use a single nymph or maybe a dry/dropper rig because, like Shawn, I feel that a double rig is often a colossal tangle waiting to happen!

I use a double set up when I am trying to pin point what the fish are taking. I use a larger b.h. 24-32 inches on the tippet then 18-20 inches below the first fly as a dropper. It takes some getting use to but I ain't usually smart enough to figure out what the fish are taking so it is sorta like a buffet for the fish.
I also fish droppers off a dry and the air knot as Hugh H. told me is difficult to master but useful and increases your changes of hooking up. I also use a dropper to increase drag, sometimes they just want it slowed down.

I hardly ever use only one fly unless I can zone in on what they taking.
c.v.

When fishing two nymphs I space them 6-8 inches apart. If fishing a dry and dropper or dry and emerger I space them about 2 feet apart. I only use the dry and emerger because I cannot see the emerger by itself (something about age I think).
I prefer to tie the dropper through the eye of the top fly but, I don't think it matters much.